Top 6 SQL Query Interview Questions and Answers

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Millicent Higgins
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1 Just my little additions, remarks and corrections to Top 6 SQL Query Interview Questions and Answers as published on by Holger Jakobs, holger@jakobs.com, Writing an article with the title mentioned above is generally a good idea. What hasn't been carried out so well will be discussed in this little article. General problem The author Javin Paul writes Most of the code is standard ANSI SQL, hence it will run as it is on any other database., but as shown below this doesn't hold water. The Generator Script It's full of special commands for Microsoft's T-SQL, starting with the very first command USE Test and the following line GO. Neither USE nor GO appear in ANSI/ISO SQL, nor do the first two words of the next line: IF and OBJECT_ID. Changed to ANSI/ISO SQL and nicely reformatted, the generator script reads as follows: -- Department table (first because Employee references it directly) CREATE TABLE Department ( dept_id INTEGER PRIMARY KEY, dept_name VARCHAR(20) ); -- Employee table (including the foreign key to Department) CREATE TABLE Employee ( emp_id INTEGER PRIMARY KEY, dept_id INTEGER REFERENCES Department, mngr_id INTEGER, emp_name VARCHAR(20), salary INTEGER ); -- alter table to add foreign key (only one, because Employee has been -- created with the reference to Department already) ALTER TABLE Employee ADD FOREIGN KEY (mngr_id) REFERENCES Employee(emp_id); -- populating department table with sample data

2 INSERT INTO Department (dept_id, dept_name) VALUES (1, 'Finance'), (2, 'Legal'), (3, 'IT'), (4, 'Admin'), (5, 'Empty Department'); -- populating employee table with sample data INSERT INTO Employee(emp_id, dept_id, mngr_id, emp_name, salary) VALUES ( 1, 1, 1, 'CEO', 100), ( 2, 3, 1, 'CTO', 95), ( 3, 2, 1, 'CFO', 100), ( 4, 3, 2, 'Java Developer', 90), ( 5, 3, 2, 'DBA', 90), ( 6, 4, 1, 'Adm 1', 20), ( 7, 4, 1, 'Adm 2', 110), ( 8, 3, 2, 'Web Developer', 50), ( 9, 3, 1, 'Middleware', 60), ( 10, 2, 3, 'Legal 1', 110), ( 11, 3, 3, 'Network', 80), ( 12, 3, 1, 'UNIX', 200); If you want to make sure that tables with the same name do not exist prior to using this script, it might be best to just drop them unconditionally. This may show up as an error, but so what! The Queries Please refer to the original article for full descriptions of questions. 1. Employees (names) who have a bigger salary than their manager SELECT a.emp_name a JOIN Employee b ON a.mngr_id = b.emp_id WHERE a.salary > b.salary; I would keep this query as is. 2. Employees who have the biggest salary in their Department SELECT a.emp_name, a.dept_id a JOIN (SELECT a.dept_id, MAX(salary) as max_salary a JOIN Department b ON a.dept_id = b.dept_id GROUP BY a.dept_id) b ON a.salary = b.max_salary AND a.dept_id = b.dept_id; This query works fine, but is far more complicated than necessary. This is my version: SELECT emp_name, dept_id e WHERE salary = ( SELECT MAX(salary) sub WHERE e.dept_id = sub.dept_id ) ORDER BY 2; In order to make the result reproducible, I added the ORDER BY 2 clause.

3 3. Departments that have less than 3 people in it SELECT dept_id, COUNT(emp_name) as 'Number of Employee' HAVING COUNT(emp_name) < 3; The construction is fine, but naming any kind of object be it user, database, table, schema, column or any other it is necessary to put double quotes around, if the name contains any non-ascii character, spaces or special characters except _. Using single quotes will not work with any standard conforming database system. Since AS is a keyword, I capitalized it. SELECT dept_id, COUNT(emp_name) AS "Number of Employees" HAVING COUNT(emp_name) < 3; 4. All Department along with the number of people there SELECT b.dept_name, COUNT(a.dept_id) as 'Number of Employee' a FULL OUTER JOIN Department b ON a.dept_id=b.dept_id GROUP BY b.dept_name; Same error as above with single/double quotes. ANSI/ISO SQL provides NATURAL joins, which work fine if columns containing the same kind of value are named the same. This is the case in the sample database (as well as in Microsoft's Northwind database), so using this feature (which is offered by all major SQL database systems, but is not included in T- SQL), the query becomes shorter and nicer. Again, I added an ORDER BY clause for reproducibility. By the way, the keywords INNER and OUTER should always be avoided, because they are just noise and have no effect whatsoever in any database. And no, they do not make the statement more legible. Beginners must learn that LEFT, RIGHT and FULL are the keywords in any outer join, and the lack of all these creates an inner join. SELECT dept_name, COUNT(emp_id) as "Number of Employee" NATURAL FULL JOIN Department GROUP BY dept_name ORDER BY 1; 5. Employees that don't have a manager in the same department SELECT a.emp_name a JOIN Employee b ON a.mngr_id = b.emp_id WHERE a.dept_id!= b.dept_id; I would keep this query as is. 6. All Department along with the total salary there SELECT b.dept_name, SUM(a.salary) as 'Total Salary' a FULL OUTER JOIN Department b ON a.dept_id = b.dept_id GROUP BY b.dept_name; Same error as above with single/double quotes. Again, using the feature of NATURAL FULL JOIN makes life easier, freeing the user from qualifying column names. If you think that

4 having a numeric zero (0) instead of a NULL value for the total salary of an empty department is nicer, the COALESCE function can be of help. SELECT dept_name, COALESCE(SUM(salary), 0) as "Total Salary" NATURAL FULL JOIN Department GROUP BY dept_name; Additional Queries Actually, the original article lists another 6 queries from earlier articles, which are also a bit intricate. I also reviewed them as well. 7. Can you write an SQL query to find the second highest salary of Employee? The first suggestion does not conform to the ANSI/ISO SQL because there is no keyword TOP. The second one is this (just reformatted for legibility): SELECT salary FROM ( SELECT salary ORDER BY salary DESC LIMIT 2 ) AS emp ORDER BY salary LIMIT 1; But actually, a subquery is not necessary: SELECT salary ORDER BY 1 DESC LIMIT 1 OFFSET 1; 8. How to find all duplicate records from a table? select name, count(name) as times from contacts group by name, phone having times>1; This query will result in an error in all standard confirming databases, because the column times is formed after the having clause is evaluated. Nice idea, but a typical error of beginners. SQL queries are not evaluated in the order the clauses are written! Therefore, a standard conforming solution would be (keywords capitalized): SELECT name FROM contacts GROUP BY name HAVING COUNT(*) > 1; 9. How do you copy all rows of a table using SQL query? create table table_name as select * from source_table; Besides capitalization of keywords, I would keep this as is. Please consider that this only creates a new table, but is not capable of adding the rows from some query to an existing table. And of course, no constraints are copied over, but just the results of the query. In order to add rows from a query to an existing table, do this: INSERT INTO table_name SELECT * FROM other_table;

5 Of course, the query can be more complex that the example. 10. How do you join more than two tables in SQL query? This is a funny question. Why should joining any number of table create a problem unless the number get really huge. The author even writes There are enough to confuse someone on SQL JOIN ranging from various types of SQL JOIN like INNER and OUTER join,..., although especially the words INNER and OUTER have absolutely no effect at all, but are just noise (already mentioned above). Of course you have to grasp the differences between inner and outer Joins, but in the syntax these words do nothing. Further down in the article, there are Venn diagrams of SQL Joins, which always confuse people, because the only set operations in SQL are not Join operations at all, but use the keywords UNION, INTERSECT and EXCEPT. 11. How to find 2nd highest salary without using a co-related subquery? select * from ( select e.*, row_number() over (order by salary desc) as row_num from Employee e ) where row_num = 2; This is a possible solution, but not as easy as the solution mentioned under number 7 above. 12. There exists an Order table and a Customer table, find all Customers who have never ordered. SELECT A.Name FROM Customers A WHERE A.Id NOT IN (SELECT B.CustomerId FROM Orders B) This is nice, but I cannot gather why alias names for the tables and thus qualified column specifiers are used. Actually, it should read like this: SELECT Name FROM Customers WHERE Id NOT IN (SELECT CustomerId FROM Orders) That's it! All my solutions have been verified and tested with the world's most advanced open source SQL database system, PostgreSQL. You can find it on the web: As opposed to Oracle and T-SQL, it supports more ANSI/ISO SQL data types like BOOLEAN, but also advanced features like long text columns up to 1 GB in size, precise numbers with more than 38 digits, arrays, real DATE, TIME, TIMESTAMP and INTERVAL values, just like XML and JSON columns. Of course, full support for PRIMARY KEYs, FOREIGN KEYs, CHECK constraints and full text search is included as well. Advanced SQL features like WINDOW functions, Common Table Expressions and full checking of input values will make life easier. The license is BSD, so you can use it for whatever you want. Auckland,

INDEX. 1 Basic SQL Statements. 2 Restricting and Sorting Data. 3 Single Row Functions. 4 Displaying data from multiple tables

INDEX. 1 Basic SQL Statements. 2 Restricting and Sorting Data. 3 Single Row Functions. 4 Displaying data from multiple tables INDEX Exercise No Title 1 Basic SQL Statements 2 Restricting and Sorting Data 3 Single Row Functions 4 Displaying data from multiple tables 5 Creating and Managing Tables 6 Including Constraints 7 Manipulating